The predistortion linearizer method based on digital signal processing (hereinafter referred to as a digital predistortion method) (for example, GB 2 335 812 A, hereinafter referred to as Patent literature 1) is known as one of the non-linear compensation method for a microwave power amplifier. The digital predistortion method is characteristic in that a complicate analog circuit is eliminated by implementing a predistorter using digital signal processing. A conventional linear amplifier such as a feed forward amplifier or a negative feedback amplifier is mainly realized by an analog circuit. The predistorter is also realized by an analog circuit (for example, H. Girard and K. Feher, “A new baseband linearizer for more efficient utilization of earth station amplifiers used for QPSK transmission”, IEEE J. Select. Areas Commun. SAC-1, No. 1, 1983, hereinafter referred to as Non Patent literature 1, and Nojima and Okamoto, “Analysis and Compensation of TWT Nonlinearities Based on Complex Power Series Representation”, the IEICE Transactions B, Vol. J64-B, No. 12, December 1981, hereinafter referred to as Non Patent literature 2). The art of the analog linearization circuits, however, generally requires a highly accurate adjustment technique. The analog circuits need to be easily configured to be simple in order to make a transmitter including a modulator more compact and more cost effectiveness.
As a conventional digital predistorter, a configuration having a lookup table for linearizing non-linear characteristics of an amplifier has been known (for example, L. Sundstrom, M. Faulkner, and M. Johansson, “Quantization analysis and design of a digital predistortion linearizer for RF power amplifiers”, IEEE Trans. Vech. Tech., Vol. 45, No. 4, pp. 707-719, 1996. 11, hereinafter referred to as Non Patent literature 3). A lookup table type predistorter reads out distortion compensation data from a lookup table by using a difference signal between the input signal of the power amplifier and the output signal of the power amplifier and adds the distortion compensation data to the input signal of the power amplifier. If distortion component is completely compensated for in the output signal of the power amplifier, i.e., the output signal of the power amplifier has been linealized, the difference signal would become nothing. In the lookup table type predistorter, the lookup table is updated in stages so that an absolute value of the difference signal is nothing. In such a manner, the lookup table type predistorter can perform distortion compensation without previously recognizing non-linear characteristics of the power amplifier.
For the existing wireless systems, for example, PDC (Personal Digital Cellular), GSM (Global System for Mobile Communications), IMT-2000 (International Mobile Telecommunication 2000) have been operated. It is known that each wireless system allocates each frequency band. On the other hand, an art for realizing software defined radio so that a single hardware accommodates a plurality of wireless systems has been known. If a single hardware can accommodate a plurality of wireless systems, a user of the single hardware can use a single mobile communication provided by a wireless system without being aware of a wireless system or a core network behind the wireless system. Such a single hardware corresponding to a plurality of wireless systems, however, has not been actually realized.
As the services provided by wireless systems differ for each area or for each operator, wireless systems required for the services will also be diversified. For that purpose, wireless systems suitable for respective objects need to be mixed in the same period of time and in the same place.
As a transmission method for using the plurality of wireless systems, a multi-band wireless system has been known. The wireless system makes use of one or more of frequency bands to be used according to a propagation environment or a traffic status variable in accommodative manner. A multi-band transmission by using an unused frequency band is effective to reserve predetermined quality or predetermined capacity of transmission. In a multi-band wireless system, the number of frequency bands used can be always changed depending on a transmission status to be secured in each wireless system. Similarly, a bandwidth can also be changed in the same band. If a frequency band used by a business entity is different from a frequency band used by anther business entity, the multi-band wireless system can improve efficiency of using a frequency by executing adaptive transmission control so as to use a free frequency band based on the technologies of interference recognition, frequency sharing, interference canceling, suppression of giving-interference, or multi-band controlling.
In the base station lookup table type predistorter accommodating such a multi-band wireless system, a frequency range to which distortion compensation is available requires generally more than three times transmission bandwidth centering on a carrier frequency. For example, a system for transmitting both signals in 800 MHz band and in 1.5 GHz band simultaneously, it is not possible to achieve predetermined distortion compensation while performing distortion compensation on the frequency band signals. Because it is known that the conventional look-up table type predistorter can operate less than 20-MHz bandwidth. Therefore, in order to transmit signals in a plurality of frequency bands, while executing distortion compensation by using a lookup table type predistorter, the transmitter needs to build a lookup table type predistorter for each frequency band and also prepare a vector modulator, a frequency converter and a power amplifier for each frequency band. That makes the entire device cumbersome, increases power consumption and overgrows a device.
If the transmitter can collectively execute a lookup table type predistortion processing on a plurality of frequency bands, it can simplify a device, suppress power consumption and downsize the device. When the transmitter is about to execute lookup table type distortion processing on a plurality of frequency bands by using a single lookup table under the condition that frequency bands are away from each other by frequency separation more than a bandwidth between respective frequency bands such as 800 MHz and 1.5 GHz, a compensation parameter stored in the lookup table would be inaccurate to signals in both frequency bands simultaneously. Therefore, distortion components produced in both frequency bands cannot be appropriately compensated for.
In a multi-band wireless system with a plurality of transmission frequency bands, it can be assumed that a frequency band may be changed in consideration of a service state of the wireless system and interference or the like to the other wireless systems. When a frequency band in such a wireless system is being changed, a conventional lookup table type predistorter that has a fixed frequency band to execute distortion compensation cannot adaptively change even an operating frequency. The lookup table type predistorter used for a long time needs to execute lookup table type predistorter modification and change at each base station for changing a frequency band. That requires tremendous time and effort to readjust many lookup table type predistorters. Therefore, a lookup table type predistorter configuration for making a transmitter more cost effectiveness by eliminating the requirements mentioned above is needed now.
For example, the lookup table type predistorter, which can execute distortion compensation in the frequencies f1 and f2 simultaneously, cannot execute the distortion compensation in the frequencies f1 and f3 simultaneously when the frequency band for signals used is changed from f2 to f3. This is because that an operating frequency band for conventional lookup table type predistorter is fixed.
Conventional lookup table type predistorters cannot adaptively change a frequency band to be subject to distortion compensation and the bandwidth, when frequency bands of a wireless system are being changed. Conventionally, it is required to update or exchange a lookup table type predistorter for each of plural frequency bands.